Pneumatic rubber tires typically contain a circumferential tread of a rubber composition having a suitable balance between abrasion resistance, rebound and tan delta (0° C.) values to promote tire treads having resistance to treadwear, good rolling resistance and having suitable wet traction.
For some applications, it has been proposed to provide such rubber compositions with an inclusion, or dispersion, of a finely divided, high density, high molecular weight polyethylene.
For example, U.S. Pat. No. 4,675,349 relates to a tire tread cap which contains high molecular weight particles having a weight molecular weight average of from one to six million, a softening point above 275° C. (133° C.) together with processing oil, wherein it is required that the polyethylene is mixed with the rubber composition at a relatively low mixing temperature of from about 200° F. to 240° F. (about 93.3° C. to 115.5° C.) which is well below the softening point of the polyethylene. The polyethylene is reported as being a crystalline and linear polymer.
For example, U.S. Pat. No. 4,005,054 relates to a rubber composition containing a crystalline polyolefin, which can be a polyethylene, having a molecular weight equal or greater than 500,000 in which a substantial portion is in a condition of fine particles which are drawn out in a form of elongated fibrils in a manner that it is said that the rubber composition has a very high resistance to elongation in at least one direction. The rubber composition is prepared by kneading at a temperature close to and preferably higher than the melting of the polyolefin so as to obtain a fibrillation in situ of the polyolefin particles.
Other patent publications relating to rubber compositions which contain an inclusion of a dispersion of a very high molecular weight polyethylene polymer, particularly for one or more tire components, include, for example U.S. Pat. Nos. 6,036,800, 3,842,883, 3,759,306 and 6,202,726 and European patent publication EP 906,838.
In practice, this invention relies upon an inclusion of m-phenylene bismaleimide by hot blending with an ultra high molecular weight crystalline polyethylene (UHMWPE) which is considered herein to enhance one or more physical properties of a rubber composition which contains the UHMWPE. For such inclusion, it is required that the m-phenylene bismaleimide is blended, together with the elastomers of the rubber composition, with the UHMWPE at a temperature above the melting point of the UHMWPE.
It is recognized that the ultra high molecular weight crystalline polyethylene particles are both particularly incompatible with the diene-based elastomer(s) contained in the tread rubber composition and, moreover, do not contain olefinic unsaturation to co-vulcanize with the elastomers in the rubber composition. While the mechanism may not be entirely understood, it is believed herein that the hot mixing (above the melting point of the polyethylene polymer) of the m-phenylene bismaleimide with both the elastomer(s) and ultra high molecular weight polyethylene polymer may act to promote crosslinks between the UHMWPE and the diene-based elastomers.
In the description of this invention, the term “phr” where used relates to parts by weight of an ingredient per 100 parts by weight of rubber, unless otherwise indicated.
The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” are used interchangeably unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless indicated.